Molto rallentando (molto rall.) in music means "greatly gradually slow down".
Word by word translation:
molto = very; much
(rall.) rallentando = gradually getting slower
The majority of musical terms are in Italian, other common languages for musical terms are German and French.
See a list of Italian Musical Terms at Music Theory.
Learn 14 Common Musical Terms All Piano Players Need to Know at TakeLessons.
Check out 4 Dos and Don'ts When Writing Songs, shared by BMI.com.